Klf4 (KruppeMike factor 4, previously known as GKLF) is an epithelial zinc-finger protein which controls cellular proliferation and differentiation in the skin and gastrointestinal tract. Klf4 mRNA is found at high evels in growth-arrested cells and is nearly undetectable in dividing cells. Klf4 is normally found only in regions of differentiating epithelial cells, including in the esophagus, and expression is downregulated in epithelial dysplasia, including polyps and cancer. Mice homozygous for a null mutation in Klf4 die on postnatal day 1 and show abnormal differentiation of the skin and colon (Katz et al, Development, 2002). Using tissue-specific gene ablation in mice, we have also demonstrated that Klf4 plays a vital role in adult gastric epithelial proliferation and differentiation (Katz et al, Gastroenterology, 2005). As Klf4 regulates a number of genes known to be important in esophageal proliferation and differentiation, including keratin 4, ED-L2, and keratin 19, Klf4 undoubtedly plays a critical role in the esophagus as well. Notably, keratin 19 is nvolved in malignant transformation in both the esophagus and pancreas. Nonetheless, the function of Klf4 in esophageal epithelial cells has not been investigated. In these studies, we will examine the role of Klf4 in the esophagus by testing the following hypotheses: (1) Loss of K!f4 in esophageal epithelia results in increased cell proliferation and/or altered differentiation, and (2) Klf4 overexpression in esophageal epithelial cells alters cellular differentiation and/or proliferation. The following interrelated Specific Aims will test these hypotheses using genetic and biochemical approaches: (1) To analyze the function of Klf4 in esophageal epithelial homeostasis (a) through studies of histology, cell proliferation, and differentiation in adult mice lacking Klf4 in the esophageal mucosa and (b) by analyzing the effects of Klf4 deletion in primary esophageal cells; and (2) To study the effect of Klf4 overexpression in esophageal epithelia by (a) overexpressing Klf4 in mice using the esophageal-specific EBV ED-L2 promoter and (b) by analyzing the effects of increased Klf4 expression in primary esophageal cells. Taken together, these mechanistically oriented and complementary studies will provide new insights into normal esophageal epithelial homeostasis and establish paradigms for how the proliferation-differentiation equilibrium may be subverted during mucosal injury and transformation.